1. Field of the Invention
The present invention relates to a servo motor controller having automatic gain adjustment function capable of automatically adjusting velocity control gain in accordance with an object to be driven.
2. Description of Related Art
In general, in a servo motor controller, in order to improve the response of the velocity control, it is important to select a suitable velocity control gain in accordance with a driven object. Since an optimal value of the velocity control gain varies with an object driven by the motor, it is necessary to adjust the velocity control gain in accordance with the object to be controlled. An exemplary method of adjusting the velocity control gain, known to those skilled in the art, is to perform analysis based on a frequency response method, and to observe the gain margin and the phase margin of the input/output gain, and based on an observed value, to set an adjustment magnification factor for an operation constant.
However, in a method in which the gain margin is observed, for example, it has been found that, when adjustment is performed by setting the adjustment magnification factor based on the ratio of the observed gain margin to the gain of −3 dB so that the input/output gain of −3 dB may be obtained at a frequency corresponding to the phase lag of −180°, the resulting input/output gain obtained by actually using the set adjustment magnification factor exceeds −3 dB. The resulting input/output gain exceeds −3 dB because the relation between the operation constant and the input/output gain of the frequency response of the velocity control is not an exact proportional relation. It has also been found that, when only the gain margin is observed, the input/output gain may become large at frequencies other than the frequency corresponding to the phase lag of −180°.
As another example of prior art, a gain adjustment method of adjusting gain of a positioning control system applied to a control system for positioning a HDD head is disclosed in Japanese Patent Publication No. H08-195043. In paragraph 0007, Japanese Patent Publication No. H08-195043 includes a description that “the system comprises detection means that output the positional error signal corresponding to the positional error between the present position and the target position of a head, and compares the amplitude of the reference measurement signal with the positional error signal and adjusts the system gain so as to bring the comparison results into coincidence.”
As still another example of prior art, a drive controller that is capable of identifying frequency characteristics is disclosed in Japanese Patent Publication No. 2003-316402. In paragraph 0056, Japanese Patent Publication No. 2003-316402 includes a description that “a servo motor is controlled by adding a specified signal (a signal consisting of sine waves of different frequencies successively outputted or a signal consisting of overlapped sine waves of plural different frequencies and of same amplitude) to a current instruction value generated from a position instruction value, and by using the addition result to control the servo motor. On the other hand, response of the servo motor (velocity feed-back value, position feed-back value, current feed-back value, or the like) is measured, and the amount of the features which is the result of a predetermined operation performed on the measurement result (difference between the maximum value and the minimum value of the response of the servo motor) is compared with a prescribed target value. When, for example, the amount of features is not within a certain range, the amplitude of the specified signal is caused to fluctuate so that the amount of features and the target value come closer to each other. After this adjustment is performed repeatedly, and when the amount of features enters into a specified range, it is judged that the frequency characteristic is obtained in sufficient precision. Thus, since the frequency characteristic can be identified by using the specified signal that has been adjusted in an optimal state, identification of the frequency characteristic can be achieved in high precision even if the feedback control system changes.”